The invention relates generally to echo receiving systems and more particularly to scan converters used to transfer the polar coordinate or range-azimuth radar video into a memory field organized in cartesian or x,y coordinates.
As is well known in the art, marine pulse radars are used to detect the presence of objects which produce echo signals from transmitted pulses. Generally, an antenna scans in azimuth while transmitting pulses. When a transmitted pulse strikes an object such as, for example, a distant ship or land, returns are received back at the antenna in time as a function of the range of the objects from which they are reflected. Accordingly, the input to the radar receiver for one transmitted pulse can be characterized as a train of echos or pulses from objects at different distances. After demodulation to remove the radar's carrier frequency, this pulse train is known as radar video. The video from each transmitted pulse is identified as data in a plurality of successive range cells or bins. The video data is therefore received in polar coordinates which means that the data in each range cell is defined by an azimuth angle and a range value.
As is well known, it is desirable to visually display the radar returns for operator interpretation. One common radar display has been the plan position indicator (PPI) wherein the video data for successive transmitted pulses are displayed in successive sweeps from the center of the polar coordinate system on a CRT with a long persistence phosphor. A serious drawback of a PPI-type display is that it is refreshed only once per revolution of the radar antenna resulting in a relatively low intensity display that is difficult to see in daylight. Because of this and other drawbacks, more recent radar systems have commonly used a raster-scan display because its refresh rate is much higher and therefore provides an intensity which can be viewed in daylight.
Because the raster scan systems generally require the data be presented to the CRT in rectangular or x,y coordinates, a scan conversion process is required to convert the polar coordinate data that is received into cartesian coordinates. In the prior art, scan converters have been used to transfer data from range-azimuth memories into memories having one-to-one or bit image correspondence with the x,y pixels or elemental display positions on a CRT. One type of prior art scan converter generates x,y and R addresses along the line defined by the azimuth angle of the present sweep so as to address x,y memory locations in a bit image memory (BIM) and, synchronously, addresses R memory locations in a memory containing the video data for the present sweep.
A method for generating x,y and R addresses has been to increment one coordinate by pixel unity and increment the other by a fraction corresponding to the trigonometric function of the side between the unity coordinate and the azimuth of the sweep. Because of the nature of trigonometric functions, it has been desirable to partition azimuth into a plurality of sectors or, more preferably 45.degree. octants, and use appropriate trigonometric functions for each octant. A problem with this approach has been that artifacts may be generated along some of the octant boundaries.
Another problem of a scan converter is that because there are a plurality of sweep lines passing through each pixel close to own-ship, the same pixel corresponding address may be generated a plurality of times during each scan. Accordingly, the video data written into a memory location for a pixel may be overwritten when that address is generated on a subsequent sweep of the same scan. Accordingly, there may be an echo return for that pixel that is erased and therefore not displayed.